The present invention relates generally to appliances used in dentistry and is more paticularly concerned with new and improved appliances used principally in orthodontics but also having application to prosthetics and oral surgery.
As is well known, orthodontic appliances are used to manipulate certain teeth to correct irregularities and/or abnormalities in their relationships with surrounding members. This is achieved by using force systems that have their origin primarily in elastically-deformed wires which absorb and release energy during loading and unloading. Heretofore, the force-imparting wires used in orthodontic treatment have been made mainly from 18-8 stainless steel wires that take advantage of the bending and torsional properties of that wire.
As mentioned in the article by C. J. Burstone et al entitled "Force Systems From An Ideal Arch," American Journal of Orthodontics, Volume 65, pages 270-289 (1974), proper application of the correct forces requires not only the study of suitably contoured and clinically dimensioned shapes or configurations together with variations in the cross-sectional dimensions of the force-imparting wire, but also a better understanding of the biomechanics involved in orthodontic appliances. Heretofore, efforts have been directed almost exclusively toward the development of optimum appliance configurations with only ancillary consideration being given to the material used for the appliances.
With respect to biomechanics, it was reported by C. J. Burstone et al in Angle Orthodontist, Volume 31, pages 1-14 (1961), that desirable tooth movement can best be achieved by producing an optimal force system capable of delivering relatively light but continuous corrective forces. The primary or basic biomechanical characteristics include a lower force magnitude whereby the teeth will move rapidly and relatively painlessly with minimum tissue damage, a constant force level over time as the appliance experiences deactivation in order to provide maximum tissue response, an accurate location of the point of application of the force or its equivalent, a uniformity in the force applied through the total distance over which the force acts. It also is desirable to provide within a orthodontic appliance the ability to undergo large deflections without deformation. Of course, if the force acting on the teeth decays too rapidly, the teeth will move more slowly and it becomes more difficult to accurately produce the desired effect.
Heretofore, the force magnitude applied to the teeth was determined in part by the cross section of the wire used in the appliance, with smaller wires providing the desired lower or reduced force. As will be appreciated, larger wires fit well in the slots of band-mounted or directly bonded brackets or the lumen in a tube and a good fit is necessary for controlled tooth movement. If smaller wires are used, the play between the wire and the bracket leads to loss of control. Reduction in slot or lumen size is undesirable since (1) it is more difficult to control tolerances and (2) manufacturing variations in wire cross section have a proportionately greater effect on force magnitudes. Despite this, a reduction in the wire cross section with its attendant reduction in load deflection rate historically has been the course followed to achieve force constancy using 18-8 stainless steel wire. In this connection, care must be taken since too great a reduction in cross section can result in permanent deformation before optimal forces are reached.
Although the principal and predominant emphasis in orthodontic research has been on improved appliance design, and relatively little attention has been given to alternatives for the conventionally employed 18-8 stainless steel wire, efforts are now being made in providing the aforementioned desirable biomechanical characteristics through the use of alternative materials. One example of such an approach can be found in the proposed utilization of Nitinol alloys of the type described in U.S. Pat. No. 3,351,463. These materials are near-stoichiometric intermetallic compounds of nickel and titanium, preferably having cobalt substituted for the nickel on an atom to atom basis. The alloy can be preformed below its critical transition temperature and, when heated to above that temperature, will display a mechanical memory causing the material to return to its predisposed shape. The application of this material to orthodontics is set forth in U.S. Pat. No. 4,037,324 where the longitudinal shrinkage characteristic of the wire is used. Although this intermetallic material is reported to be quite ductile, it has been found in practice that the material will not withstand cold bending into major orthodontic configurations and cannot be used for closing loops and the like. This, of course, severly limits the alloy's use in the formation of appliances that require significant bends in their design. Additionally, the material cannot be welded or soldered, thereby substantially hampering its utilization.
The present invention solves many of the problems encountered heretofore when using stainless steel or Nitinol while, at the same time, facilitating the delivery of optimum orthodontic forces. Accordingly, it is an object of the present invention to provide a new and improved orthodontic appliance utilizing an orthodontic force-imparting wire that provides optimum orthodontic force characteristics including the preferred low force magnitude and force constancy over a prolonged period of time to achieve continuous, relatively painless tooth movement with maximum tissue response and minimum tissue damage. Included in this object is the provision for a force-imparting wire exhibiting a low modulus of elasticity relative to the 18-8 stainless steel wires used heretofore.
Another object of the present invention is to provide a new and improved orthodontic appliance of the type described that facilitates the application of a given force with greater ease and accuracy and exhibits the ability to undergo larger deflections without deformation, therefore providing an associated increase in the effective working time of the appliance while meeting the necessary criteria of biocompatability, formability and environmental stability.
Yet another object of the present invention is to provide an orthodontic appliance of the type described that utilizes a new and improved force-imparting wire exhibiting a lower modulus of elasticity, a greater maximum elastic deflection and a greater ratio of yield strength to modulus of elasticity while reducing the need for periodic installation of wires of reduced cross section. Included in this object is the provision for the use of a wire of moderate cross section, thereby minimizing the need for closer wire tolerances for achieving desired first order deflections. Also included is the provision for wires whose force magnitudes and moment to force ratios are controlled by selection of the modulus of elasticity rather than the traditional approach of simply modifying the cross section.
Still another object of the present invention is to provide a new and improved orthodontic appliance of the type described that utilizes room temperature stabilized beta-titanium alloys capable of being formed into a wide array of orthodontic appliances from the simple to the highly complex orthodontic configurations in order to deliver the optimum moment to force ratios such that the appliance acts on the crown of the tooth to provide the accurate center of rotation of the tooth as it is moved. Included in this object is the provision for the use of a stabilized beta-titanium material capable of taking advantage of the excellent plasticity and formability characteristics associated with its beta crystalline structure yet, at the same time, providing the desired strength characteristics by controlling the mechanical and thermal history of the solid solution alloy.
A still further object of the present invention is to provide a new and improved orthodontic appliance of the type described which exhibits the ability to be welded to base or main arches or segments thereof without materially affecting the mechanical properties of the appliance. For example, hooks, tiebacks, ligature wires and springs made from the beta-titanium alloy wire can be directly welded, unlike the 18-8 stainless steel that requires an elaborate and time consuming soldering process which negatively influences wire properties such as yield strength. The weldability of the material also facilitates greater flexibility of use wherein lighter control wires can be rigidly secured to the heavier wires to assure proper anchorage for the delivery of optimum magnitude forces at more constant force rates.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are achieved in accordance with the present invention by providing an orthodontic appliance that utilizes a force-imparting wire formed from a room temperature stabilized beta-titanium alloy having a modulus of elasticity well below 20.times.10.sup.6 psi. The titanium alloy wire provides a ratio of yield strength to modulus of elasticity up to 80% greater and more than that of 18-8 stainless steel wire of the same cross section and exhibits a higher maximum elastic deflection than such stainless steel wire. The titanium alloy wire further is characterized by an ability to provide a lower and more constant force component over a longer period, thereby enhancing the effective working time of the appliance and by an ability to withstand repeated cold bending into major orthodontic configurations.
A better understanding of the invention will be obtained from the following detailed description and the accompanying drawing of illustrative applications of the invention wherein the features, properties and relation of elements are described and exemplified.